chapter 5, what holds the atmosphere up? module six

Chapter 5, What Holds the Atmosphere Up? Module Six

How the greenhouse effect works within the temperature structure of Earths atmosphere The greenhouse effect is powered by the lapse rate Atmospheric scientists call the change in temperature of the air with altitude the lapse rate It is about 6C colder per kilometer of altitude The lower part of the atmosphere is called the troposphere

Atmosphere The troposphere is the lower part of the atmosphere It contains about 90% of the air It contains all of the weather The boundary of low temperature is about 17 km high on average The boundary where the air temperature reaches its coldest point is the tropopause Commercial airplanes fly in the tropopause

Atmosphere with altitude

Atmospheric layers Troposphere about 10 km high, contains 90% of air and all of the weather Tropopause boundary where air is the coldest, commercial aircraft area Stratosphere air begins to warm up because of ozone content Mesosphere not much effect on the weather Exosphere ditto

No temperature contrast, no greenhouse effect Remember the layer model with a skin temperature Think of the skin altitude for the air column as some kind of average altitude from which the IR escapes to space The idea of a skin layer in the atmosphere is fuzzier than using a glass pane in the layer model but it still a useful concept

Lapse rate vs. strength of GH effect If we increase the GHG concentration of the atmosphere, the IR radiation to space will originate from a higher altitude (skin altitude). The increase in skin altitude increases the ground temperature. If the temperature of the atmosphere was the same at all altitudes, then raising the skin temperature would have no impact on the ground temperature. More CO2 higher skin altitude warmer ground

Pressure as a function of altitude The pressure in the atmosphere depends primarily on the weight of the air over your head The weight of the overhead air at sea level is more than The weight of the overhead air at the top of a mountain The pressure of the air is non-linear with altitude (unlike scuba diving, where the pressure is linear with depth)

Pressure is a non-linear (exponential) function

What to remember When a gas is depressurized (less pressure) the gas expands When a gas expands, it cools When you pressurize a gas it heats up

Expansion, Compression and Heat If we had a gas inside a container with a piston, and pressurized the gas, it would heat up, even in an insulated container with no heat entering or leaving A closed system with no heat coming in or out is called adiabatic If gas is compressed adiabatically, it warms up. It takes work to compress a gas, the work energy is transferred to heat When it expands, it cools, reversing the process and the gas cools down

Water vapor and latent heat Remember chemistry and the phase change diagram, where energy is added, the substance stayed at the same temperature until it completely changed phase, solid to liquid, or liquid to gas. The energy that was added is called latent heat Latent heat of fusion between solid and liquid Latent heat of vaporization between liquid and gas In one direction the heat is added, in the other direction the heat is released.

Phase changes Solid + heat liquid (latent heat of fusion) melting Liquid + heat gas (latent heat of vaporization) boiling When the phase change goes in the other direction, the same amount of energy is released during condensation or freezing Vapor liquid + heat released Liquid solid + heat released

Latent heat You charge up an air parcel with latent heat when you evaporate water into it (vapor contains the latent heat- not sensible heat) You get the heat back when the water condenses and the latent heat is released A thermometer does not measure latent heat A thermometer measures sensible heat (what you can sense)

Equilibrium conditions When water is in equilibrium between liquid and vapor, its called saturated, or 100% relative humidity, and the equilibrium vapor pressure of water will be high. Undersaturated occurs when it is cold, the amount of water vapor is lower than the equilibrium value Supersaturated occurs when vapor pressure is higher than equilibrium, and the vapor tends to condense into precipitation

Convection Convection occurs when you heat a fluid from below or cool it from above (either a liquid or a gas) Fluid expands as temperature increases, density decreases Unstable condition causes the fluid column to turn over Warm fluid rises to the top The Atmosphere tends to mix when it convects

Air is compressible The air is not all the same temperature Pressure is higher at the bottom because o f the weight of the air column Compressed air at the bottom heats up Because the air is well mixed, the moving air will always find itself at the same temperature as the rest of the air in the column This is what static stability looks like in a column of compressible air the same temperature as the rest of the column

Convection in the atmosphere Driven by sunlight hitting the ground Warms the air at the bottom of the column Warm air begins to rise, as it rises, it expands, and cools While ascending, it remains lighter and warmer than the air around it If it does not mix on the way up, the air can get all the way to the top of the column If it mixes on the way up, the whole column warms up uniformly

Moist Convection The latent heat in water vapor drives most of the drama in our weather

Moist convection Air at the surface of the Earth with a relative humidity of 100% rises due to convection As the temperature drops, the equilibrium amount of water vapor decreases Supersaturation drives water to condense into droplets or ice The story of cloud formation will continue in chapter 7

Water vapor It changes the temperature of the air It systematically changes the lapse rate Dry convection has a lapse rate of about 10C temperature change per km of altitude Add the latent heat in moist convection, the lapse rate decreases to about 6C per km It is possible that the lapse rate of the atmosphere could be different in a changing climate

Take home points, Chapter 5 Air in the upper troposphere is colder than air at the ground because of the process of moist convection. The process includes the following: Convection is driven by sunlight heating the air near the ground The air rises and cools because it expands Water vapor condenses, releasing heat as the air rises

Continued The moist convecting air gets colder with altitude, but not as much as if it were dry If the air did not get colder with altitude at all, there would be no greenhouse effect

Revisit the layers of the atmosphere Troposphere Stratosphere Mesosphere Entering outer space: Ionosphere Exosphere

Chapter 6, Weather and Climate How the Weather Affects the Climate

Chaos 10 days is the limit for predicting weather because weather is chaotic an extreme sensitivity to initial conditions, so that small differences between two states tend to amplify, and the states diverge from each other The butterfly effect, a puff of air from a butterflys wing eventually resulting in a giant storm somewhere that would not have happened if the butterfly had never existed

Butterfly effect First observed in a weather simulation model The model stopped running Edward Lorenz restarted it by typing in the variables like temperature and wind speed He had small, insignificant changes, such as rounding errors The model diverged completely from the results of the initial simulation

Edward Norton Lorenz Mathematician Edward Norton Lorenz was an American mathematician and meteorologist, and a pioneer of chaos theory. He discovered the strange attractor notion and coined the term butterfly effect. WikipediaWikipedia Born: May 23, 1917, West Hartford, CT BornWest Hartford, CT Died: April 16, 2008, Cambridge, MA DiedCambridge, MA Books: The essence of chaos BooksThe essence of chaos Education: Massachusetts Institute of Technology, Dartmouth College, Harvard University EducationMassachusetts Institute of Technology Dartmouth CollegeHarvard University

Weather Forecasts rely on computer models Small imperfections in the initial conditions and the model cause the model weather to diverge from the real weather By about 10 days the prediction is worthless To overcome the error, run the model may times with tiny variation in initial conditions an ensemble of model runs

Climate Defined as some time average of the weather Climatological January (or any other month) would be the average of many Januaries The weather is chaotic, but the climate generally is not The weather would predict rain on a particular day, whereas the climatologist may predict a rainy season

Averaging Layer Model Real World Warm and cold Summers and winters Day and night Completely balanced energy budget Averaging is valid Some places much hotter Some place much colder Radiative energy budget at some place could be wildly out of balance Will averaging change the answer to something unreasonable?

Averaging a non-linear system Top panel averaging radiative energy flux (S-B equation) over a large temperature range introduces a large bias. Bottom panel over the temperature range of normal Earth conditions, the blackbody radiation energy flux is closer to linear, so averaging over a small range would be less of a problem

The Fluctuating Heat Budget Not stable like a model, but fluctuates widely Solar energy comes in only during the day Sunshine varies seasonally and by location Infrared is radiated day and night The energy budget is in balance over a 24 hour period But at any time in any spot on the planet, the energy is usually out of balance

Seasonal variations The seasons are caused by the tilt of the Earth relative to its orbit around the sun, the obliquity Wintertime, days are shorter and the sun is lower Added over a day the winter hemisphere has less sunlight

Seasons are NOT caused by the Earths distance from the sun The eccentricity cycle refers to the shape of the Earths orbit around the sun It varies from elliptical, to circular Currently we are in a near circular orbit The Earth is actually closer to the sun in January than it is in July Seasons are not caused by proximity to the sun

Earths seasons are caused by the tilt of the poles relative to the orbit, and not by its distance to the Sun

Incoming flux depends on latitude and day of the year Northern hemisphere summer is in the middle of the plot, which shows flux as a function of latitude and time of the year.

Interesting to note from the plot Highest daily fluxes are at the poles during the summer Poles get six months of sunlight Sun whirls around in a circle above the horizon (not overhead) Why isnt it a tropical garden in the summer?

Thermal Inertia Damps out the temperature swing between day and night Damps out the temperature swing as the seasons change Even damps out the temperature change of global warming

Oceans Has a tremendous capacity to absorb and release heat from the atmosphere Land not so much diffusion through the soil is slow and only affects the first meter or two Cool water surface turns over and has convective mixing to about 100 meters Maritime areas have milder seasons Middle of large continents have more intense seasonal cycles

Averaging a seasonal cycle Out of balance because of the heat distribution from the water and from the wind The outgoing heat in the tropics cant keep up with the incoming solar radiation The heat is carried to cooler, higher latitudes by water and winds The Earth can vent the excess heat to space from the higher latitudes

Heat carried to higher latitudes for venting to space

The Coriolis Acceleration http://www.youtube.com/watch?v=i2mec3vgeaI http://www.youtube.com/watch?v=aeY9tY9vKgs http://www.youtube.com/watch?v=iqpV1236_Q0 Two clips on the Coriolis Effect and one shows a Foucault Pendulum, demonstrating the rotation of the Earth.

Coriolis Effect The water and the air feel the most effect at the poles (incredibly high tides in higher latitudes, nearly no tide difference at the equator) At the equator there is no apparent rotation The middle latitudes fall somewhere between these two extremes

Modeling the Weather Fluids are governed by Newtons Laws of Motion because fluid has mass and inertia Inertia is the sluggishness of matter to resist changes in motion Tendency to keep moving if its moving, or remain stationary if it is already stationary To change speed or direction, motion requires a force such as gravity or a change in pressure (weather)

Bathtub vs. Earth Bathtub flows more quickly than the Earth rotates, so does not feel the Earths rotation Flows in the atmosphere and ocean persist long enough to feel the effect of a rotating Earth Ocean flows can be driven by friction with the wind Coriolis acceleration tries to deflect the flow to the right in the northern hemisphere After a few rotations, a steady state is reached where the fluid flows 90 degrees to the wind

The eventual steady state Top the fluid initially flows in the direction of the wind. Middle after a while the Coriolis force swings the fluid to the right. Eventually, the fluid itself flows 90 degrees to the wind or pressure force, and the Coriolis force just balances the wind or the pressure force. Bottom the steady state where the flow stops changing and remains steady.

Geostrophic Flow In a rotating world the fluid will eventually end up flowing completely crossways to the direction that Its pushed. This condition is called geostrophic flow. A geostrophic flow balances the forces on it against each other.

Geostrophic cells on weather maps Cells of high pressure and low pressure with flow going around them Low pressure, pressure force points inward, 90 to the right of that the winds flow counterclockwise in the N. hemisphere cyclonic direction of flow High pressure, pressure force points outward, and the flow is clockwise around the high pressure anticyclonic direction

Surface wind field from a climate model (computer generated)

Parameterization, assumptions in models Assume that cloud formation is a function of humidity in the air, humidity is a parameter that would control cloudiness Effects of turbulent mixing Air-sea processes such as heat transfer Biology modeling

Take home points chapter 6 The energy budget to space of a particular location on Earth is probably out of balance, fluctuating through the daily and seasonal cycles and with the weather, This is in contrast to the Layer Model. The annual average energy budget for some location on Earth may not balance either, because excess heat from the tropics is carried to high latitudes by winds and ocean currents. The global warming forecast requires simulating the effects of weather, which is a really difficult computational challenge.

Chapter 7, Feedbacks Complexity in the Earth system arises form the way pieces of it interact with each other

Positive and Negative Feedbacks A feedback is a loop of cause and effect At the center of a feedback is a state variable (average temperature of the Earth) A positive feedback makes the temperature change larger than it would be without the feedback A negative feedback counteracts some of the external forcing, and tends to stabilize the state variable

Feedbacks: A positive feedback is an amplifier A negative feedback is a stabilizer

Stefan-Boltzmann Feedback Negative feedback a stabilizer The radiated infrared heat attempts to pull the temperature back down

Ice Albedo Feedback Positive feedback an amplifier Ice albedo feed works on the state variable of temperature. An input perturbation, such as a rise GHG, drives temperature up. Ice melts, reducing the albedo, and warming the ground up a bit. The direction of the input and the feedback loop agree with each other. It can also go in the other direction, perturbation cools things down and feedback agrees.

Water Vapor Feedbacks Positive Negative Water is involved in a positive feedback loop acting on global temperature Warming allows more water to evaporate before it rains Water vapor is a GHG Doubles the climate impact of rising CO2 concentrations Without the water vapor feedback, climate would be less sensitive to CO2 There is a negative feedback loop that controls the amount of water vapor in the atmosphere at any given temperature, having to do with rainfall and evaporation (the hydrological cycle)

At the center of the feedback loop is a state variable

Runaway Greenhouse Effect It is possible for the water-vapor feedback to feed into itself Means the end of a planets water Earths climate uses the high latitudes as cooling fins to avoid the runaway greenhouse effect A runaway greenhouse effect stops if the vapor concentration in the air reaches saturation with liquid water or ice, so that any further evaporation would just lead to rainfall or snow

Phase diagram shows that Venus had a runaway GH effect, but not Earth and Mars Triple point of water Pressure: 0.006207 atm Temperature: 0.01C (273.16 K)

Earth retained its water Earth has its water because of the structure of the atmosphere The tropopause acts as a cold trap, making sure that water vapor rains or snows out before getting too close to space The oceans are protected by a thin layer of cold air for billions of years now The Hadley circulation controls the distribution of atmospheric water vapor warm air rises at the equator, it cools and water condenses

Clouds Cirrus high altitude thin and wispy, barely noticeable, and made of ice crystals Cumulus clouds storm clouds are towers, the result of focused upward blasts of convection Stratus clouds low clouds layered, formed by broad diffuse upward motion spread out over large geographical areas

Clouds: Interfere with both incoming visible light, and outgoing IR light In the IR, clouds act as blackbodies, warming the planet Incoming visible light is reflected back to space, cooling the planet The overall impact of a cloud depends on which of these two effects is stronger, which in turn depends on what type of cloud it is

Earths Energy Budget The difference between Earths energy budget between absorbed and scattered sunlight is that when light is scattered back to space, its energy is never converted to heat, so it never enters into the planets heat budget

Clouds: Vary by meteorological conditions and human pollution Cloud droplet size is an important factor The smaller the drop, the better it scatters light Rain clouds look dark because they have large droplets, and are optically thick Cloud droplets are affected by cloud condensation nuclei (seeds) that help droplets form Sea salt, pollen, dust, smoke, and sulfur compounds from phytoplankton

Human footprints Sulfate aerosols from coal fired power plants Internal combustion engines Forest fires, heating fires and cooking fires Contrails (short for condensation trails) jet airplanes passing through clean air containing water vapor Persistent spreading contrails are thought to have a significant effect on global climate

Generalities - You cant see through low clouds meaning they are optically thick You can see through high clouds, optically thin High clouds warm, low clouds cool Clouds that form in dirty air tend to be better light scatterers with a higher albedo, cooling the planet Clouds are the largest source of uncertainty in climate models

Ocean Currents, el Nio climate oscillation Periodic flip flop between two states of the ocean called el Nio and la Nia Ocean interaction with the atmosphere, corresponding atmospheric cycle called the Southern oscillation ENSO el Nio Southern Oscillation The state of the ENSO affects climate patterns around the world

El Nio climate oscillation La Nia El Nio Cool surface water Productivity high Fisheries good Equatorial E W wind Tilted thermocline Wetter weather Warm surface water Less fertile Fisheries collapse Winds diminish Thermocline collapses Drier weather

Meridional overturning circulation in the North Atlantic Gulf stream carries warm water from tropics to the North Atlantic Water cools and sinks, making more room for warm water Greenland ice cores show instability in Meridional overturning synchronous with large temperature swings (~ 10C) within a few years 8.2k event (8200 years ago) catastrophic freshwater release to the North Atlantic Circulation will slow down with melting ice

Terrestrial Biosphere Feedbacks Changes in vegetation could alter the albedo of the land surface when ice melts Land surface stores carbon Trees evaporate water through transpiration (a self- replicating cycle) Droughts, vegetation dies, soil dries, and the water shortage is a positive feedback

Carbon Cycle Feedbacks The subject of the next three chapters (Module 7)

Feedbacks in the Paleoclimate Record Models tend to under-predict the extremes of climate variation in the real world climate The future may surprise us

Take home points chapter 7 Positive feedbacks act as amplifiers of variability, whereas negative feedbacks act as stabilizers. The water-vapor feedback doubles or triples the expected warming owing to rising CO2 concentrations. The ice albedo feedback amplifies the warming in high latitudes by a factor of three or four.

Continued Clouds have a potentially huge impact on climate. Clouds are expected to exert an amplifying feedback to climate warming, although the strength of this feedback is uncertain. Clouds are the largest source of uncertainty in model estimates of the climate sensitivity.

chapter 5, what holds the atmosphere up? module six

Documents

altitude slide

air temperature

skin temperature

higher altitude skin

tropopause slide

troposphere slide

ground temperature

depth slide